PHOTOINITIATOR SELECTION FOR FOOD PACKAGING INKS AND COATINGS – CONCERNS AND OPPORTUNITIES

UV curing inks and coatings are specially formulated products that remain in liquid form until they are cured with exposure to intense ultra-violet radiations. Those UV radiations cross-link the ink components into a tough polymer. The cross-linking of the ink molecules makes them durable. The rate of this reaction is really fast - almost instant. UV inks are better than classical inks. As these inks contain no solvents, and don't rely on absorption into substrates for drying. 

By far, the biggest advantage to UV curing ink is lack of solvent or volatile organic compounds. In addition, UV inks don't contain any solvent that is evaporated, so 100% of the ink deposit is colorant. Also, they don't absorb into porous materials, they sit on the surface. This provides more colour intensity to the printed material, even on using less ink. Since the inks can only be reacted with strong UV light, they don't clog heads or screens like solvent-based inks, allowing for more production and less maintenance. Instant drying means prints are cured and ready to be finished immediately after printing. UV curable inks, coatings and varnishes possess several advantages over other ink chemistries.

The UV printing inks & coatings/ varnishes comprises of four major components- monomers, oligomers, pigments, and photo initiators. The monomers provide a building block of the ink, and can contribute certain properties such as softness/hardness of the inks when cured, as well as flexibility or elongation characteristics of the inks & coatings for varying types of applications. Monomers also help control viscosity of ink & coatings, essential to jetting reliability depends on functionality and molecular structure of monomer.

The reactive oligomers acts as resins in the ink formulation and consist uniquely formulated adhesive components for printing on a wide range of different substrates.

Monomers provide the solvent like properties in the ink and together with oligomers react chemically to form the binder system.

Pigments/ Colorants and additives used in UV-curing inks correspond approximately to those of other printing ink technologies.

Photo-initiators (PI) are the most important components of UV printing inks and coatings since these substances are responsible for capturing that energy from photons. When UV printing inks exposed to UV curing process, photo initiators are activated by UV radiation (wavelength 100 - 380 nm) and decomposed to radicals and thereby release energetic species that can initiate the process of polymerization. 

The UV photo curing process is based on the absorbance of UV irradiation. The overall procedure of polymer formation after receiving UV energy by the photo initiator has four stages: initiation, propagation, chain transfer and termination. There are three major types of photo-initiators:

Type I: When UV based inks or coatings formulation exposed to UV light, radiations are absorbed by the type I photo initiators which undergo a homolytic cleavage process, via Norrish type I reactions (photochemical cleavage or homolysis of aldehydes and ketones arising two free radical intermediates) to create free radicals that initiate the polymerisation process. Cleavage occurs at any weak bond, but usually takes place at α position of carbonyl group (α-cleavage), and less probably is the β-cleavage. Examples of some Type I free-radical photo initiators includes, α- aminoalkylacetophenones (AAAP’s) such as methyl-1-(4-methylthio)phenyl-2-morpholinopropan-1-one (MMMP) and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)- butan-1-one (BDMB); hydroxyacetophenones (HAP’s); benzylketals such as 2,2-dimethoxy-2-phenylacetophenone (BDK); phosphine oxides etc.

Type II: When UV light incident on these photo initiators, these substances get excited but do not induced free radicals unlike type I photo initiators. This is because CO–aryl bond energies are too high to be broken by UV energy, so the presence of a co-initiator is necessary which may function in two different ways either hydrogen abstraction (from an H donor compound, or the environment), or through electron transfer (from an electron donor and subsequent proton transfer step). Examples, Benzophenones, one of most commonly used PIs worldwide due to their characteristics and low cost, with strong absorption between 230-260nm and weak around 330nm. Their properties make them an excellent choice alone or in combination with other photo initiators. There are also many derivatives with strong absorption in the 280-330nm range and similar reactivity to type I photo initiators. Thioxanthones, with maximum absorption at 380-420nm. They are extensively used alone or with other photo initiators, having the advantage not produce photo-yellowing, and also their excellent behavior as sensitizers.

Cationic photo initiators have garnered popularity in past few years due to their advantages like insensitivity to oxygen quenching, very low shrinkage, excellent adhesion and resistance allow to producing more uniform and stable polymers, over the free-radical photo initiators. Sulphonium salts, usually triarylsulphonium salts & Iodonium salts, usually diaryliodonium salts are the most commonly used cationic photo initiators for food packaging.

However, the use of UV & energy curable printing inks for food packaging application is critical as both selection of raw materials and curing processes are not as simple as with conventional solvent-based and water-based technologies. The reason being few incidents occurred in past, such as in 2005, the findings of isopropyl thioxanthanone (ITX) in baby milk packaged in beverage cartons in Europe, that triggered the issue of migration of low-molecular weight photo-initiators (usually < 1000 Daltons) into food from UV printing inks and varnishes used for food packaging. Most of current photo initiators have a molecular weight under 500 Daltons (the vast majority around 250 Daltons or less) and the scission products are smaller. This incident led to the recall of over 30 million liters of infant formula throughout Europe.  This incident alerted Rapid Alert System for Food and Feed (RASFF) which further reviewed other food products and found more than 100 incidents within few years related to migration of substances from food contact material into food. In 2009, ten RASFF notifications were issued regarding the migration of the PI’s benzophenone and 4-methylbenzophenone. These notifications led to the recall of several batches of breakfast cereals and nearly 7 tons of milk across Europe. Between 2000 and 2011, 143 notifications were issued by RASFF in the European Union regarding the migration of PI’s from food packaging into foodstuffs

Novel Technology

When exposed to UV energy, UV-photo initiators (PI’s) present in the formulations produce free radicals which catalyse polymerization of monomers and pre-polymers into resins. In addition to photopolymerization, other free radical reactions occur in these systems resulting in the formation of chemically varied photolytic decomposition products, many of which are low molecular weight chemical species ( usually less than 1000 daltons) with high migration potential. The risk of migration of photo initiators markedly increases when decreasing their molecular weight. In last few years significant work has been done to overcome the drawbacks of conventional photo initiators.

The introduction of the so called ‘multifunctional photo initiators’ (MFPI) with several photoactive groups, usually two, linked to a relatively small core unit. In this way, it is possible to avoid the reduction of reactivity because the content of photoactive groups in the large molecule is in a similar range as for low molecular weight PIs. Apart from the expected lower migration of these polymeric MFPI, due to their higher molecular weight, they also have fewer and lower volatility photodecomposition byproducts and a higher probability than conventional PIs of being bound into the cured polymer matrix, hence decreasing the level of unreacted materials that are able of migrating. Examples of some commercially available polymeric multifunctional photo initiators includes, Polybutylene glycol bis(4-benzoylphenoxy)acetate (CAS No: 515136-48-8); Bis(benzophenone-2-carboxylic acid) polyethylene glycol ester (CAS No: 1246194-73-9); Poly(ethyleneglycol) bis(p-dimethylamino benzoate) (CAS No: 71512-90-8); Benzophenone tetramer-type (CAS No: 1182753-56-5); Polyol ester-type (CAS No: 1182755-49-2) etc.

Alternatives to design “Low Migration UV curable inks & Coatings)"

• Development of high molecular weight polymeric and oligomeric photo initiators or macro- photo initiators having molecular weight >1000Da with no toxicological concern. These molecules should have high reactivity and solubility, low volatility, not release odorous compounds; and to be without photo-yellowing effects. The easiest way to avoid the migration of harmful PIs into food.
• Another feasible alternative is the use of “self-initiating resin” in UV ink formulation in place of photo initiators. To increase the efficiency and photo-polymerisation rate of these processes new approaches based on charge-transfer systems are being developed.
• Another option is “Multicomponent Photo initiating Systems” with three components or even with four components. The components increases the efficiency of the photo initiating system which is being proven very useful in high speed cures.
• The use of electron beam curing technology over UV curing can also be a potential alternative in future in the ink industry.

How to select the correct photo initiators to design UV curable inks & coatings for food applications?

The selection of right type of photo initiator is a very critical process since the photochemical and photophysical properties of each photoinitiator have an impact on the final product (structure, cured speed, by-products, etc.). The following factors should be assessed before selecting PIs to formulate UV inks & coatings for food packaging applications.

• The maximum absorption peak of chosen PI at the wavelength emitted by the UV lamp and a high quantum yield.
• The solubility of the photoinitiator affects the stability of the formulation.
• High storage stability and easy handling of the molecule.
• The photoinitiator should not stimulate the generation of odours, volatile compounds or toxic photoproducts. Neither should the photoinitiator cause any yellowing or discoloration on the final surface of the polymer.
• Two essential aspects must be minimised: the migration of photocuring components or by-products and, the economic cost of the process.

If the equitable photo initiators are chosen appropriately, UV curable inks and coatings would be an ideal option for food packaging applications. 

Analytical procedures available for migration testing of photo-initiators from food packaging materials into the food products.

The main analytical techniques used for the determination of photon initiators are chromatographic techniques such as HPLC & GC. These analytical techniques offers the opportunity to perform different methods depending on its objectives and requirements, using both a nonspecific and specific detector such as flame ionisation (FID) and mass spectrometric (MS) or tandem mass spectrometric detectors (MS/MS) respectively. For more accuracy, HPLC & GC techniques can be used in combination with Mass spectroscopy (MS) such as LC-MS & GC-MS. Last but not the least, the another useful analytical technique reported is Direct Analysis in Real Time (DART) coupled to a mass spectrometer (DART-MS).